Update postgresql.conf to increase max_slot_wal_keep_size

[encima]

What kind of change does this PR introduce?

Increases max_slot_wal_keep_size default from 1GB

What is the current behavior?

Current behaviour is 1GB by default.

As this is a setting we do not expose, the current default is too low to successfully replicate using a tool like Airbyte (as the WAL will grow too large during the Hourly sync window).

What is the new behavior?

Proposing to set this as 4GB by default but 2GB could be a conservative option.

Additional context

Some relevant issues are noted in the (internal) Notion document